In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of a Device-to-Device (D2D) proximity service is discussed as a new function on and after Release 12 (see Non Patent Document 1).
The D2D proximity service (D2D ProSe) is a service in which direct device-to-device communication is provided within a synchronization cluster formed by a plurality of user terminals which are synchronized to one another. The D2D proximity service includes a discovery procedure (Discovery) in which a proximal terminal is discovered and D2D communication (Communication) that is direct Device-to-Device communication.
A discovery procedure, in which a user terminal that exists in a first cell discovers a proximal terminal that exists in a second cell which is provided around the first cell, is called an inter-cell discovery procedure (Inter-cell discovery). D2D communication in which a user terminal that exists in the first cell performs communication with a proximal terminal that exists in the second cell is called inter-cell D2D communication (Inter-cell communication).
However, in a case where a frequency employed in the first cell and a frequency employed in the second cell differ, even when the user terminal transmits a discovery signal in order to discover the proximal terminal in the Inter-cell discovery, the proximal terminal is not capable of receiving the discovery signal. Therefore, in such a case, a procedure in which the Inter-cell discovery is appropriately performed is desired.